Sixteen postal workers in Atlanta have been sentenced to between three and nine years in prison after being convicted of accepting bribes as low as $250 to deliver cocaine on their mail routes, officials said Tuesday.

Federal agents learned of the deliveries in 2015 while investigating a drug trafficking ring in the city, the U.S. Attorney for the Northern District of Georgia said in a news release. The mail employees were targeted by traffickers because they believed the workers were less likely to draw attention from law enforcement, the release said.

“Postal employees are paid to deliver mail, not drugs,” said Imari R. Niles, Special Agent in Charge, U.S. Postal Service Office of Inspector General, Capital Metro Area Field Office. “The vast majority of the Postal Service’s 600,000 employees are hard-working, trustworthy individuals."

Agents used a confidential source to pose as a drug trafficker looking for postal workers willing to deliver cocaine and marijuana, and recorded the interactions. The workers chose to deliver cocaine instead of marijuana, believing they could charge a higher bribe to make the deliveries, the release said.

"When the confidential source asked if they knew any other postal workers who did the same thing, some of the defendants introduced the confidential source to coworkers who also wanted to deliver packages (with the defendant claiming an additional bribe for every package their recruit delivered), the release states.

The workers range in age from 26 to 64 and all must pay $1,450 to $10,500 in forfeiture or restitution.